1. Fair and Protected DLS September 2007 Date:2007-09-18
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2. Outline Simulation scenario Simulation results (Cases 1-7)
September 2007
Outline
Simulation scenario
Simulation results (Cases 1-7)
RTS/CTS approach
DLA approach
Poll?

3. September 2007
Simulation scenario
Two direct links QSTA 1-> QSTA 2 and QSTA 3-> QSTA 4
All QSTAs use Block ACK transmission
Initiators of direct transmissions work in saturation
11a PHY, 54 Mbps
QSTAs are moving as it is shown in the slide 2 1 3 4

4. Case 1: OK
September 2007
All four QSTAs are in TX-range of one another
Existing methods are good enough

5. Case 2: EIFS instead of DIFS
September 2007
QSTA 3 sends data to QSTA 4
When QSTA 4 replies with an ACK, QSTA 1 senses noise
QSTA 1 waits EIFS, while QSTA 3 waits only DIFS<EIFS, before backoff
QSTA 3 has an advantage over QSTA 1

6. Case 3: unfair collision
September 2007
QSTA 1 and QSTA 3 send RTS frames at the same time
It is a collision for QSTA 2, but not for QSTA 4
QSTA 4 replies with an ACK, and QSTA 3 starts a data transmission
QSTA 3 has an advantage over QSTA 1

7. Case 4: unfair collision + EIFS instead of DIFS
September 2007
Both problems as in Cases 2 and 3

8. Case 5: unfair CW growth September 2007
QSTA 3 starts a data transmission, QSTA 2 sets its NAV
QSTA 1 sends an RTS, but QSTA 2 does not reply
QSTA 1 increases its CW, performs backoff and sends the RTS again, and again…
… QSTA 1 drops the planning data packet, CW of QSTA 1 is huge

9. Case 6: unfair CW growth - EIFS instead of DIFS
September 2007
The same problems as in Cases 2 and 5

10. Case 7: spatial reuse benefit
September 2007
Pairs of QSTAs are absolutely hidden from each other: spatial reuse is possible
More than good: total throughput is more than 54Mbps

11. Summary of simulation analysis
September 2007
Summary of simulation analysis
Problems: unfair distribution of channel capacity
Using EIFS instead of DIFS
Unfair growth of CW
Unfair collisions
Opportunities: using of spatial reuse benefit
Not to lose it while resolving problems

12. RTS/CTS approach September 2007 This approach: Simple Standard frames
Reliable protection of direct links
RTS
CTS
But it does not resolve the problem of using EIFS instead of DIFS (e.g., see Case 2)
DATA
But…

13. RTS/CTS approach September 2007 But it allows NO spatial reuse
3->AP RTS
AP->3 CTS (all STAs receive)
The CTS from AP prohibits all other transmissions
However, transmissions 1->2 and 3->4 would be possible at the same time
DATA
RTS
CTS
DATA

14. Direct Link Announcement
September 2007
Direct Link Announcement
We propose to introduce a special Direct Link Announcement (DLA) data frame
Basic idea: the source QSTA shall broadcast/multicast the DLA frame over the AP by setting ‘To DS’ bit to 1
DLA frame includes:
Direct transmission source and its destination
Duration of the planning direct transmission (a-lá NAV)
DLA expiration time; after the time, DLA should be ignored

15. DLA September 2007 Protection:
All QSTAs are informed of the planning direct transmission and its duration
Upon receiving DLA back, QSTA 3 starts DATA after PIFS without contention
Fairness:
DLA resolves problems:
Using EIFS instead of DIFS
Unfair growth of CW
Unfair collisions
AP queues the DLA
DLA
DLA
ACK
ACK
DATA

16. DLA: option 1 September 2007 Spatial reuse :
During DL Setup or by other radio measurement procedures, QSTAs collect a list of neighbors which it may interfere with
If the list is included into the DLA payload, spatial reuse is possible
QSTAs which are not in the list may communicate during the direct transmission without sending DLA frames
DATA
DLA 5
DATA

17. DLA: option 2 September 2007 In addition to DLA, RTS/CTS approach
Aim: to protect direct links from legacy STAs
DLA
DATA 5
RTS
CTS
DATA

18. September 2007
DLA + option 1 vs. Legacy

19. September 2007
Straw poll
Should we prepare the normative text on Direct Link Announcement (DLA)?
Yes:
No:
Abstain: